Adding extension attributes on boundaryEvents

we need to add some extra attributes on a BPMN we are generating from a config file. We tried to add some ExtensionAttributes (we're using the org.activiti.bpmn.model package API) on some boundaryEvents (timer events we placed on some userTasks), but when producing the XML with org.activiti.bpmn.converter.BpmnXMLConverter such attributes are not serialized in the XML.

Is that a bug or a feature (joking of course, I mean, is that on purpose?)